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Notes from the Field

Trail Breaking Grand Mesa

Ground validation measurements are an important part of working with remote sensing instruments like SWESARR. Snow is dynamic and changes occur as soon as it settles to Earth’s surface. In a dry, windy place like Colorado’s Grand Mesa, snow can move around, be packed into drifts, get intercepted in canopy, sublimate back into the atmosphere, and metamorphose into depth hoar as winter progresses.

Ludo Brucker (NASA GSFC / Universities Space Research Association) and Chris Hiemstra (Cold Regions Research and Engineering Laboratory) were on Grand Mesa preparing for NASA SnowEx 2019. During the latter half of November 2018, Grand Mesa was blanketed in 40-70 cm (about 15-27 inches) of new snow from a series of storms. Fortunately, a short break in the weather with clear skies was forecast for the last SWESARR engineering flight. Optimistic plans to take early season measurements on Grand Mesa materialized.

This image shows the sky around the ultra diffuse galaxies NGC 1052-DF4 and NGC 1052-DF2. It was created from images forming part of the Digitized Sky Survey 2. NGC 1052-DF2 is basically invisible in this image.  In 2018 an international team of researchers using the NASA/ESA Hubble Space Telescope and several other observatories uncovered, for the first time, a galaxy in our cosmic neighbourhood that is missing most of its dark matter. This discovery of the galaxy NGC 1052-DF2 was a surprise to astronomers, as it was understood that Dark matter (DM) is a key constituent in current models of galaxy formation and evolution. In fact, without the presence of DM, the primordial gas would lack enough gravity pull to start collapsing and forming new galaxies. A year later, another galaxy that misses dark matter was discovered, NGC 1052-DF4, which further triggered intense debates among astronomers about the nature of these objects. Now, new Hubble data have been used to explain the reason behind the missing dark matter in NGC 1052-DF4, which resides 45 million light-years away, providing further evidence for tidal disruption. By studying the galaxy’s light and globular cluster distribution, astronomers have concluded that the gravity forces of the neighbouring galaxy NGC 1035 stripped the dark matter from NGC 1052-DF4 and are now tearing the galaxy apart.

The only problem was that the flight lines were well out into the west end of Grand Mesa and the early season snow was new and deep. Chances of getting stuck on the way out there were high, especially as the route went uphill from Grand Mesa Lodge perched on the south side of Crag Crest. Fortunately, Grand Mesa Lodge owner Mike Renner grooms Grand Mesa snowmobile trails throughout the winter season and he groomed a route out to the west side of the Mesa the previous evening. On the morning of December 4, Ludo and Chris rode 11 miles to the west end of the Mesa.

Just as they arrived in the blazing sun, the drone of the Twin Otter carrying SWESARR and flying one of the lines could be heard. They cleaned out the radar reflectors, then set about collecting snow data. The corner reflectors are used to assess the airborne radar observations.

Ludo measured snow density and temperature, as well as snow stratigraphy and grain size information.

In all, five snow pits were performed and over 1500 snow depths were measured at various points along several pre-determined flight lines.

After a week of snowfall, welcome clear and calm conditions resulted in an outstanding day collecting ground-truth data useful for assessing instruments and algorithms for the early-season snowpack.